The research examines: (a) the neurophysiological control of normal laryngeal functioning in respiration, swallow, and speech; (b) the pathophysiology of laryngeal function in voice disorders; and (c) the development and evaluation of new treatment approaches for voice disorders. a) The effects of hypercapnia on laryngeal muscle activation during quiet respiration was studied in 6 normal volunteers and found to increase activation levels in adductor, tensor and abductor laryngeal muscles. In another study in normals, higher cross correlations were found during phonation between adductor and tensor muscles on the same side of the larynx than between the same muscle on opposite sides of the larynx. Such coordination between laryngeal muscles on each side of the larynx may provide similar length, tension and adduction characteristics regardless of probable anatomical asymmetry between the right and left vocals folds. b) Using bipolar needle EMG electrodes in patients with symptom recurrence following recurrent laryngeal nerve resection, no significant difference in percent activation were found between the thyroarytenoid muscles on the surgical and nonsurgical sides. Also, the percent activation levels in both thyroarytenoid were greater than normal in these patients during respiration and speech suggesting that reinnervation and hypertonicity of the thyroarytenoids might be associated with symptom recurrence in spasmodic dysphonia. c) Three studies were completed demonstrating that unilateral botulinum toxin injections were beneficial in particular types of patients: those with spasmodic dysphonia symptom recurrence following recurrent laryngeal nerve resection; those with vocal fold tremor primarily during exhalation; and, those with constant overadduction on the vocal folds during speech. Those with only spasmodic movements and no constant over-adduction were least benefitted by unilateral injections of botulinum toxin.